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STUDENT NOTES · DAY 9 OF 24
Build Your Own Spells 🛠️
print() and len() are spells someone else wrote. Today YOU write spells — called functions.
1def — define a spell
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Py: def writes the recipe. Nothing cooks until you CALL it: greet("Sana"). Recipe first, dinner later.
2return — the spell sends something BACK
The Function Machine: double(x)
double(5) → 🛠️ → 10
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Bittu: A function is a vending machine: coin in (argument), snack out (return value).
3The mystery of None
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Bugly: What if the function has NO return? What comes back? Hehehe…
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Professor Hoot: No return → the function quietly hands back None ("nothing"). It shouted, but returned nothing. Common beginner surprise!
4Default values — the spell's standby mode
5Quick quiz! 🧠
Which keyword creates a function?
💡 def, short for define. Python keeps ceremonies short.
def f(x): print(x) — what does y = f(3) store in y?
💡 It PRINTS 3, but with no return statement it hands back None.
6Your mission 🚀
Mission 1: Write cheer(name) that prints "Go name go! 📣" — call it for 3 friends.
Mission 2: Write square(x) that RETURNS x*x. Print square(7).
Mission 3: Write greet(name, lang="en") that says Hello for en and Namaste for hi.
Day 9 done — spellwright! 🛠️
Next: *args, **kwargs and one-line lambda spells.
Go to Day 10: Advanced Functions →